Groundwater: Characteristics and Importance

• Groundwater from wells and springs has been a key source of domestic water since ancient times.
• In the United States, more water systems utilize groundwater than surface water.
• 35% of the total population served by public water systems rely on groundwater.
• Groundwater facilities outnumber surface water installations by 10 to 1.
• Groundwater plays a vital role in supporting surface sources, sustaining dry-season stream flow.
• Groundwater's widespread distribution and desirable characteristics make it a primary water source.
• Groundwater offers a naturally purer, cheaper, and more reliable supply than surface water.
• Using groundwater eliminates substantial transmission costs and the necessity of impoundment works.
• Groundwater is economical, even when produced in small quantities.
• Engineers investigate the possibility of developing groundwater, considering factors such as effective water content, ability to transmit water, suitability of water quality, and supply reliability.
• The zone of saturation is the key subsurface water portion for a permanent, reliable supply.
• In the saturation zone, interstices are filled with water under hydrostatic pressure, moving from recharge to discharge areas.

Porosity and Effective Porosity

• Porosity affects the amount of groundwater stored in saturated materials, representing the ratio of void space to total volume.
• Porosity includes primary interstices created during rock origin and secondary interstices from geological forces.
• Secondary interstices result from geologic, mechanical, and chemical forces, such as joints, faults, fissures, solution channels, and bedding planes.
• Secondary porosity is vital in hard rocks lacking intergrain porosity, dependent on local conditions and varying with depth.
• Porosity is static and not a measure of perviousness or permeability, which affect the dynamic flow.
• Only interconnected interstices allow flow, while isolated openings hold immobile water, termed specific retention or dead storage.
• The portion of pore space allowing flow is the effective porosity or specific yield.
• Specific yields range from 0% for plastic clays to over 30% for uniform sands and gravels.
• Aquifers typically yield from 10-20%.
• Molecular and surface tension forces hold water in place, known as dead storage or specific retention.
• Effective porosity, or specific yield, is the portion of pore space where flow occurs, defined as the water proportion that can drain or be withdrawn via gravity.

Permeability

• Permeability is a rock's capacity to transmit fluid with a hydraulic gradient.
• An important factor affecting permeability is the geometry of pore spaces and rock particles.
• The pore system determines flow resistance, not relative volume.
• There is no direct relationship between permeability and porosity.
• Clays with porosities of 50% or more have extremely low permeability.
• Sandstones with porosities of 15% or less may be quite pervious.
• The standard unit of permeability is darcy (D), expressed as flow in cubic centimeters per second, of fluid with viscosity, through a 1 cm² area, under a pressure gradient of 1 atm/cm.
• Homogeneity and isotropy refer to the spatial distribution of permeability, where isotropic media have equal permeability in all directions.
• Anisotropy is common in sedimentary deposits where permeability across the bedding plane is only a portion of that parallel to the bedding plane.
• Aquifers with secondary porosity are nonhomogeneous.
• Nonhomogeneity and anisotropy effects can be analyzed under certain conditions.

Groundwater Geology

• The geologic framework provides an overview of groundwater availability.
• Rocks include hard, consolidated formations and loose, unconsolidated materials.
• Igneous rocks are intrusive and extrusive types, differing in hydrologic properties.
• Fresh intrusive rocks are compact, non-water-bearing, and have low porosity (less than 1%).
• When fractured and jointed, intrusive rocks may develop porosity and permeability within a few hundred feet of the surface.
• Metamorphic rocks are compact, crystalline, impervious, and poor aquifers.
• Aquifers store and transmit significant water quantities.
• Aquicludes contain water but do not transmit significant amounts.
• Aquitards have permeabilities between aquifers and aquicludes.
• Sedimentary formations include shale, sandstone, limestone, clay, gravel, and sand.
• Partially cemented or fractured sandstones have very high yields.
• Nonuniform distribution of interstices in limestones affects its water-bearing capacity.
• Consolidated rocks offer water to smaller areas in U.S., with most developments lying in granular, unconsolidated sediments.
• Unconsolidated sedimentary aquifers include marine deposits, river valleys, alluvial fans, coastal plains, glacial outwash, and dune sand.
• Sands and gravels are the best for water production.
• Uniform sands and gravels are the most productive.
• Clays and silts are poor aquifers due to very low permeabilities but confine water movement in pervious soils.

Groundwater Situation in the United States

• Geologic and hydrologic conditions vary greatly in various regions of the U.S.
• Thomas (1952) and McGuiness (1963) divided the United States into major groundwater regions.
• The Water Resources Division (WRD) of the US Geological Survey carries out ground water investigations.

Types of Aquifers

• Aquifers consist of unconfined, confined or artesian, and semiconfined or leaky.
• Unconfined aquifers have an upper surface under atmospheric pressure, free to rise and fall due to storage changes.
• Confined aquifers feature flow confined by an impervious layer, with water levels in wells rising above the confining layer to the piezometric level.
• Leaky aquifers are overlain or underlain by aquitards.

Groundwater Movement

• Groundwater is in constant motion.
• The movement rate is governed by frictional resistance.
• The head difference is the driving force.
• Groundwater flow velocity is low.
• Turbulent flow may occur in cavernous limestones and volcanic rocks, or coarse gravels.
• Depending on permeability, flow rates can vary within the same geologic formation.
• Flow is concentrated in zones of higher permeability.
• In homogeneous, isotropic aquifers, dominant movement is in the direction of greatest slope of the water table or piezometric surface.

Darcy's Law

• Darcy's law describes the flow of water through porous media.
• v = K(dh/dl) = KI, where v is superficial velocity, I is the hydraulic gradient, and K is hydraulic conductivity.
• Effective velocity accounts for point-to-point variations.
• K has the dimensions of velocity.
• The intrinsic permeability is k = Cd².

Aquifer Characteristics

• Transmissivity (T=Kb) characterizes the ability of an aquifer to transmit water, where b is the saturated section’s thickness and K is conductivity.
• Transmissivity (T = Kb) represents the flow via a vertical strip of aquifer extending the full thickness under a unit hydrulic gradient.
• Storage coefficient is the water volume a unit decline in head releases from storage in a unit prism of aquifer area.
• S = θγb[β + (α/θ)] for confined aquifers.
• Values of S range from 0.00005 to 0.0005 in most confined ones.
• Hydraulic diffusivity D = T/S.

Well Hydraulics

• Well hydraulics predicts yields and effects of pumping on aquifers. The factors include aquifer and operation types, aquifer characteristics, aquifer boundaries, and well construction.
• Early pumped water comes from aquifer storage near the well, creating a cone of depression.
• The cone shape changes as it expands outward, until it stabilizes.
• The drawdown curve may extend to areas of natural discharge or recharge.
• Equilibrium occurs when natural discharge decreases or recharge increases, matching the withdrawal rate.

Nonsteady Radial Flow

• Solutions for unsteady radial flow offer insight into aquifer characteristics.

Confined Aquifers

• In an effectively infinite artesian aquifer, well discharge occurs via storage reduction.
• A main equation is:

s= (Q/4πT) ∫∞u (e⁻ᵘ/u) du = (Q/4πT)W(u), with u = (r²S)/(4Tt)

Semilogarithmic Approximation

• With conditions met, Theis equation is as follows: s = Q/(4πτ) ln[(2.25Tt)/(r²S)] (SI units) s = Q/(4πT) In[(2.25Tt)/(r²S)] (SI units

Aquifer Boundaries

• Most analysis methods assume infinite aquifers, but all aquifers actually have them.
• Recharge boundaries are reproduced by introducing a negative image well.